Personal computers (PCs) that support the various Video Graphics Array (VGA) display modes commonly use a commercially available VGA application specific integrated circuit (VGA ASIC) to generate video signals from data stored in video memory. The VGA ASIC outputs three analog video signals, one for each of the colors red, green and blue. The red, green and blue output signals are provided on a set of RGB lines that connect the RGB pins of the VGA ASIC to a standard 15-pin display connector. The red, green and blue video signals are passed to a color monitor via a monitor cable, and control the red, green and blue electron beams of the monitor. When a monochrome monitor is connected to the PC, only the green signal is passed to the monitor (i.e., no connection is made between the monitor and the red and blue signal lines).
Commercially available VGA ASICs normally support one or more display modes that are compatible with monochrome monitors. To permit automatic selection of an appropriate display mode, a monitor-sense circuit is included within the VGA ASIC. The monitor-sense circuit determines the monitor type (typically following a system reset) by effectively measuring the DC resistance on each of the RGB signal lines. This is done by placing a known DC current on each RGB line and measuring the DC voltage on the line. Alternatively, the monitor-sense circuit may place a known DC voltage on each line and measure the resulting current. When an RGB line from the ASIC is not connected to the monitor (as is the case for the red and blue RGB lines when a monochrome monitor is connected to the PC), the DC resistance measured by the monitor-sense circuit is equal to the resistance of a termination resistor connected at the PC end (or "ASIC end") of the RGB line. When the RGB line is connected to a monitor, the DC resistance seen by the monitor-sense circuit is approximately equal to the resistance of the termination resistor at the ASIC end in parallel with a termination resistor at the monitor end. The monitor-sense circuit can thus determine which, if any, of the RGB lines from the ASIC are connected to a monitor, and thus determine whether a monitor is monochrome or color. Once this determination is made, a BIOS routine or dedicated hardware can be used to select an appropriate display mode.
By definition, VGA lines are 75 .OMEGA. transmission lines. Thus, in order to match the AC impedances of both the VGA monitor cable and the termination at the monitor end, each RGB line must have a 75 .OMEGA. termination at the ASIC end. A number of VGA chip manufacturers, however, have erroneously designed their VGA ASICs to work with 150 .OMEGA. current sense pull-down resistors on the RGB lines. According to "engineering folklore," this design error is the result of a mistake made by IBM in the early 1980s, in which IBM used 150 .OMEGA. termination resistors on a VGA video board schematic. The error was quickly copied by the industry, and remains as a feature of a variety of commercially available VGA ASICs.
Failure to use 150 .OMEGA. pull-down termination resistors with these erroneously designed ASICs can cause the monitor-sense circuit to fail by affecting the DC current or voltage induced during the monitor sense operation. Personal computer manufacturers that use these ASICs have therefore chosen to use the recommended 150 .OMEGA. pull-down resistors on the RGB lines to assure that the ASIC will correctly sense the monitor type, ignoring the impedance discontinuity that results on each RGB line. These impedance discontinuities cause signal reflections that reduce the quality of the color signals received by the monitor. The impedance discontinuities also increase the radiated emissions from the RGB lines.